Gas turbine engine and foil bearing system

ABSTRACT

A gas turbine engine including a compressor, a turbine and a static structure is disclosed herein. The gas turbine engine further includes a foil bearing system operative to transmit rotor loads from at least one of the compressor and the turbine to the static structure. The foil bearing system includes a foil bearing and a self-aligning foil bearing mount to align the foil bearing with an axis of rotation of the compressor and the turbine. A snubber operative to transmit rotor loads to the static structure in parallel with the foil bearing is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication 61/290,852, filed Dec. 29, 2009 and is a continuation ofU.S. patent application Ser. No. 12/974,961, filed Dec. 21, 2010, bothof which are incorporated herein by reference.

GOVERNMENT RIGHTS

The present application was made with United States government supportunder contract no. N00014-04-D-0068-002, awarded by the United StatesNavy. The United States government may have certain rights in thepresent application.

FIELD OF THE INVENTION

The present invention relates to gas turbine engines, and moreparticularly, to a gas turbine engine foil bearing system.

BACKGROUND

Foil bearing systems in gas turbine engines remain an area of interest.Some existing systems have various shortcomings, drawbacks, anddisadvantages relative to certain applications. Accordingly, thereremains a need for further contributions in this area of technology.

SUMMARY

One embodiment of the present invention is a unique gas turbine engine.Another embodiment is a unique gas turbine engine foil bearing system.Other embodiments include apparatuses, systems, devices, hardware,methods, and combinations for gas turbine engines and gas turbine enginebearing systems. Further embodiments, forms, features, aspects,benefits, and advantages of the present application shall becomeapparent from the description and figures provided herewith.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying drawingswherein like reference numerals refer to like parts throughout theseveral views, and wherein:

FIG. 1 schematically illustrates a gas turbine engine in accordance withan embodiment of the present invention.

FIG. 2 schematically illustrates a foil bearing system in accordancewith an embodiment of the present invention.

FIG. 3 depicts a skewed relationship between an axis of rotation of agas turbine engine rotor and a centerline of a foil bearing.

DETAILED DESCRIPTION

For purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings, and specific language will be used to describe the same.It will nonetheless be understood, that no limitation of the scope ofthe invention is intended by the illustration and description of certainembodiments of the invention. In addition, any alterations and/ormodifications of the illustrated and/or described embodiment(s) arecontemplated as being within the scope of the present invention.Further, any other applications of the principles of the invention, asillustrated and/or described herein, as would normally occur to oneskilled in the art to which the invention pertains, are contemplated asbeing within the scope of the present invention.

Referring now to the drawings, and in particular, FIG. 1, a non-limitingexample of a gas turbine engine 10 in accordance with an embodiment ofthe present invention is schematically depicted. In one form, gasturbine engine 10 is an axial flow machine, e.g., an air-vehicle powerplant. In other embodiments, gas turbine engine 10 may be a radial flowmachine or a combination axial-radial flow machine. It will beunderstood that the present invention is equally applicable to variousgas turbine engine configurations, for example, including turbojetengines, turbofan engines, turboprop engines, and turboshaft engineshaving axial, centrifugal and/or axi-centrifugal compressors and/orturbines.

In the illustrated embodiment, gas turbine engine 10 includes acompressor 12 having a plurality of blades and vanes 14, a diffuser 16,a combustor 18, a turbine 20 having a plurality of blades and vanes 22,and a shaft 24 coupling compressor 12 with turbine 20. Combustor 18 isin fluid communication with compressor 12 and turbine 20. Turbine 20 isdrivingly coupled to compressor 12 via shaft 24. Turbine 20 is supportedradially by a foil bearing system 26. Although only a single spool isdepicted, it will be understood that the present invention is equallyapplicable to multi-spool engines. The number of stages of blades andvanes 14 of compressor 12, and the number of blades and vanes 22 ofturbine 20 may vary with the application, e.g., the power outputrequirements of a particular installation of gas turbine engine 10. Invarious embodiments, gas turbine engine 10 may include one or more fans,additional compressors and/or additional turbines.

During the operation of gas turbine engine 10, air is received at theinlet of compressor 12. Blades and vanes 14 compress the air received atthe inlet of compressor 12. Diffuser 16 is positioned downstream ofcompressor 12. Diffuser 16 reduces the velocity of the pressurized airdischarged from compressor 12. After having been compressed anddiffused, the air is discharged from diffuser 16 into combustor 18. Thepressurized air is then mixed with fuel and combusted in combustor 18.The hot gases exiting combustor 18 are directed into turbine 20. Turbine20 extracts energy from the hot gases to, among other things, generatemechanical shaft power to drive compressor 12 via shaft 24. In one form,shaft 24 is coupled to compressor 12 and turbine 20. In otherembodiments, shaft 24 may be coupled to only one of compressor 12 andturbine 20, and may be integral with the other. In still otherembodiments, shaft 24 may be integral with both compressor 12 andturbine 20. In one form, the hot gases exiting turbine 20 are directedinto a nozzle (not shown), and provide a thrust output for gas turbineengine 10. In other embodiments, additional compressor and/or turbinestages in one or more additional rotors upstream and/or downstream ofcompressor 12 and/or turbine 20 may be employed, e.g., in single ormulti-spool gas turbine engines.

Foil bearing system 26 is operative to react and transmit rotor loadsfrom a rotor to another structure. In one form, foil bearing system 26is operative to react and transmit loads from a rotor to a static enginestructure. In other embodiments, foil bearing system 26 is operative toreact and transmit loads from one rotor to another rotor. Rotor loadsmay include radial and thrust loads resulting from rotor weight andinertial loading, as well as pressure/thrust loading and dynamicloading. In the illustrated example, foil bearing system 26 reactsradial loads from turbine 20, whereas another bearing system B supportscompressor 12 and reacts radial and thrust loads. In other embodiments,foil bearing system 26 may support compressor 12 in addition to or inplace of turbine 20. In one or more of various embodiments, foil bearingsystem 26 may react radial and/or thrust loads for all or part of anyrotor system of a gas turbine engine such as engine 10.

Referring now to FIG. 2, a non-limiting example of foil bearing system26 in accordance with an embodiment of the present invention isschematically depicted. Foil bearing system 26 includes a foil bearing28, a bearing housing 30, a self-aligning foil bearing mount 32 and twooffset snubbers 34. In one form, foil bearing system 26 is operative totransmit loads from a rotor 36 to a static engine structure 38, such asan engine case structure or other rotor support structure. In one form,rotor 36 is a turbine rotor. In other embodiments, rotor 36 may be acompressor rotor. In one form, shaft 24 is considered to be part ofrotor 36, and in such embodiments, foil bearing system 26 may bepositioned about shaft 24 to react rotor 36 loads via shaft 24. In otherembodiments, foil bearing system may be positioned to react loadsdirectly from compressor 12 and/or turbine 20 directly. In one form,foil bearing system 26 is positioned adjacent to turbine 20 in order toreact to the turbine 20 loads and transmit the loads to static enginestructure 38. In other embodiments, foil bearing system 26 may bepositioned adjacent to compressor 12 or in other locations to react andtransmit loads from compressor 12 and/or other rotating components ofengine 10. In still other embodiments, foil bearing system 26 may reactand transmit rotor loads from one rotor to another rotor of a gasturbine engine.

Foil bearing 28 is a gas bearing. In one form, foil bearing 28 is acompliant foil air bearing. In one form, foil bearing 28 includes a bumpfoil 40 and a plurality of hydrodynamic foils, referred to herein as atop foils 42. In some embodiments, a plurality of bump foils 40 may beemployed in foil bearing 28. In some embodiments, only a single top foil42 may be employed. In one form, top foils 42 are preloaded againstrotor 36, e.g., using a spring (not shown). In other embodiments, topfoils 42 may not be preloaded, or may be preloaded by virtue of theshape of each top foil 42. Other types of foil bearings may be used inother embodiments. Bump foil 40 and top foils 42 are disposed withinhousing 30.

Rotor 36 forms a journal employed by foil bearing 28. Rotation of enginerotor 36 generates a hydrodynamic air film between rotor 36 and top foil42. The hydrodynamic air film thickness and load bearing capacityincrease with the rotational speed of rotor 36. During startup of engine10, top foil 42 rubs against rotor 36 until the hydrodynamic air filmpressure is sufficient to overcome the supported rotor 36 loads and anypreload. At normal operating speeds, the hydrodynamic air film separatesrotor 36 and top foil 42, thereby preventing contact between rotor 36and top foil 42 during normal engine operation. The hydrodynamic airfilm supports engine rotor 36. Rotor 36 loads are transmitted throughthe hydrodynamic air film to top foil 42. Top foil 42 is supported bybump foil 40, which transmits the loads to housing 30, and also providesadditional compliance to foil bearing 28. The loads are transmitted fromhousing 30 to static structure 38 via self-aligning foil bearing mount32.

In one form, self-aligning bearing mount 32 is a rigid structure. Inother embodiments, self-aligning bearing mount 32 may be configured toachieve a desired compliance and/or accommodate varying degrees ofthermal expansion. For example, in some embodiments, self-aligningbearing mount 32 may have a cross-sectional shape configured to functionas a spring, and/or may incorporate one or more springs in order toachieve the desired compliance and/or accommodate anticipated thermalexpansion. Self-aligning bearing mount 32 includes a crown 44 thatextends radially outward from bearing housing 30. Crown 44 includes acrown surface 46. In one form, crown surface 46 is a load bearingsurface. In one form, crown surface 46 is spherical. Crown surface 46may be an interrupted surface, e.g., such as embodiments wherein the tipof crown 44 is cylindrical or another non-spherical shape. In otherembodiments, crown surface 46 may not be spherical, but may be any shapehaving spherical and/or non-spherical portions that are suited to theparticular application. In one form, crown 44 is integral with housing30. In other embodiments, crown 44 may be formed separately and affixedto housing 30.

Self-aligning foil bearing mount 32 also includes a receiver 48 coupledto foil bearing 28 via crown 44 and housing 30. Self-aligning foilbearing mount 32 is formed of both crown 44 and receiver 48. Receiver 48is disposed in static structure 38. In one form, receiver 48 is split toas to allow assembly with crown 44. In one form, receiver 48 isinstalled into static structure 38. In a particular form, receiver 48 issecured in static structure 38 by a threaded nut 38A. Other embodimentsmay employ other means of securing receiver 48. In still otherembodiments, receiver 48 may be partially or fully integral with staticstructure 38. In some embodiments, receiver 48 may be anti-rotated bymeans not shown (as opposed to the threaded nut 38A in the illustratedembodiment which can be used to provide sufficient force to hold thesplit receiver 48 captive as would be understood by one skilled in theart).

Receiver 48 includes a receiver surface 50 in sliding contact with oneor more portions of crown surface 46. In one form, receiver surface 50is a load bearing surface. In one form, receiver surface 50 isspherical. In other embodiments, receiver surface 50 may not bespherical, but may be any shape having spherical and/or non-sphericalportions that are suited to the particular application. Receiver surface50 may be an interrupted surface. Crown surface 46 and receiver surface50 are operable to slide relative to each other. In the form ofspherical surfaces, crown surface 46 and receiver surface 50 permitdisplacement in the form of rotation of crown 44 and housing 30 about anaxis that is perpendicular to the rotation of rotor 36. The rotationresults from crown surface 46 sliding against receiver surface 50. Insome embodiments, crown 44 and/or bearing housing 30 may includeanti-rotation features, e.g., such as one or more pins 45 (or slots)that engage respective mating slots 47 (or pins) in receiver 48 orstatic structure 38 to prevent the rotation of housing 30 about the axisof rotation of rotor 36.

Crown 44 is a movable component of mount 32, whereas receiver 48 is astatic component of mount 32. In one form, mount 32 is a sphericalbearing. Self-aligning foil bearing mount 32 is operative to align foilbearing 28 with the axis of rotor 36. In particular, self-aligning foilbearing mount 32 is operable to self-align by displacing crown 44relative to receiver 48. More particularly, crown 44 is operable torotate in a direction perpendicular to the axis of rotation of rotor 36to self-align bearing 28. In one form, the rotation of the movablecomponent (which, in the present non-limiting example is crown 44) isrotation of the entirety of the movable component, i.e., as opposed to aflexible mount component that flexes to allow a rotation of part of theflexible mount structure. Hence, the self-alignment of self-aligningfoil bearing mount 32 is not achieved by flexure of mount 32 or any ofits components.

The operation of foil bearing 28 is dependent upon maintaining thehydrodynamic air film between rotor 36 and top foil 42 to preventcontact between rotor 36 and top foil 42 during normal engine 10operation. In order to generate the hydrodynamic air film, it ispreferable that the axis of rotation of rotor 36 be aligned with thegeometric centerline of bearing 28, e.g., so that the hydrodynamicloading on top foil 42 and the air film thickness are generally uniformalong the operating length of bearing 28, e.g., the left to rightdirection in the depiction of FIG. 2.

Referring now to FIG. 3 in conjunction with FIGS. 1 and 2, an axis ofrotation 52 of rotor 36 and a geometric centerline 54 of bearing 28 aredepicted as being skewed with respect to each other at an angle Φ.Referring to FIG. 3, the convex crown 44 rotates about an axis ofrotation 53 that is perpendicular to the axis of rotation 52 of therotor 36. In FIG. 3, the axis of rotation of the convex crown 44 isrepresented by a plus sign, indicating that the axis 53 is going intothe paper. The skewed axes result in a non-uniform loading on top foil42 and a non-uniform hydrodynamic air film thickness along the operatinglength of bearing 28, which reduces foil bearing 28 performance. It ispreferable that the skew angle Φ be small or zero so as to promote auniform hydrodynamic air film pressure along the operating length ofbearing 28 to maximize the load bearing capacity of foil bearing 28.Various factors may adversely effect the angular relationship betweenreceiver surface 50 and centerline 52. For example engine 10 componenttolerances may generate misalignment between the radial positions ofbearing Band foil bearing 28, resulting in a non-zero skew angle Φ.

Because crown surface 46 is permitted to slide against receiver surface50, self-aligning foil bearing mount 32 has only a limited ability toreact moment loading. The ability of mount 32 to react a moment is basedon the amount of friction between surfaces 44 and 50. The amount offriction may be controlled by various means, including controlling thetightness or looseness of the fit between surfaces 44 and 50, as well asby selection of the materials and/or coatings used on crown 44 andreceiver 48. In one form, the amount of friction is controlled, bydesign, to allow the non-uniform loading on top foils 42 that resultsfrom skew angle Φ to be sufficient to overcome the frictional load andimpart rotation of crown 44 relative to receiver 48 to reduce skew angleΦ. In one form, the amount of reduction of skew angle Φ is basedprimarily on the compliance of foil bearing 28 and the friction betweensurfaces 44 and 50, and may vary with the application. In someembodiments, loads transmitted from rotor 36 to snubbers 34 due to skewangle Φ may be employed to impart rotation of crown 44 relative toreceiver 48 to reduce skew angle Φ. In various embodiments,self-aligning foil bearing mount 32, e.g., crown 44 and receiver 48, maybe dimensioned so as to provide a desired operating clearance. In someembodiments, compliant springs may be employed to provide a positivefit. In some embodiments, coatings may be employed, e.g., on crownsurface 46 and/or receiver surface 50, e.g., to reduce friction andwear, depending on the needs of the particular application. One exampleof a suitable material for use as crown surface 46 and/or receiversurface 50 in some embodiments is Graphalloy®, available from theGraphite Metallizing Corporation of Yonkers, N.Y., USA.

Snubbers 34 are operative to transmit rotor 36 loads to static structure38. In one form, snubbers 34 are configured to limit the deflection ofbump foil 40, e.g., to prevent or reduce damage to bump foil 40 duringdynamic loading events. In one form, snubbers 34 include openings orslots (not shown) to permit cooling air to pass through snubbers 34 andbump foil 40. In other embodiments, cooling air may be provided tosnubbers 34 and/or bump foil 40 via one or more other schemes inaddition to or in place of openings or slots in snubbers 34. In stillother embodiments, cooling air may not be provided. In one form,snubbers 34 are operative to transmit rotor 36 loads to static structure38 in parallel with foil bearing 28, thereby sharing the rotor loadswith foil bearing 28. In one form, the rotor loads are radial loads. Inother embodiments, snubbers 34 may be structured transmit thrust loadsin addition to or in place of radial loads. In some embodiments,snubbers 34 may not be employed. In embodiments that employ snubbers 34,snubbers 34 share the rotor 36 loads with foil bearing 28 undertransient operating conditions. For example, when foil bearing 28 designloads are exceeded, snubbers 34 rub against rotor 36 to react rotor 36loads. Snubbers 34 may be formed of metallic and/or composite materials.Although two snubbers 34 are depicted in FIG. 2, it will be understoodthat in other embodiments, any number of snubbers 34 may be employed. Insome embodiments, only a single snubber 34 may be employed, e.g., at oneend of bump foil 40 and/or top foil 42; or in between a split bump foil40 and/or a split top foil 42. In various embodiments, one or moresnubbers 34 may be positioned opposite rotor 36 behind top foil 42,e.g., whereby transient rotor 36 loads are first transmitted through topfoil 42, and then from top foil 42 to snubber(s) 34. In one form,snubbers 34 are made from a carbon based material, such as steel. Inother embodiments, a carbon-fiber composite may be employed. Thematerial for snubbers 34 may vary with the needs of the application, andin various embodiments may be, for example, any suitable metallic,intermetallic and/or composite material. In some embodiments, one ormore coatings, e.g., such as alcrona (AICrN) and/or other coatings maybe employed on snubbers 34, e.g., to reduce friction and wear, dependingon the needs of the particular application. In some embodiments,snubbers 34 may include a portion made from, layered with and/orotherwise treated with a low friction material, an example of which isGraphalloy®. In other embodiments, other materials that meet therequirements of the particular application may be employed. Designconsiderations include operating temperatures, friction characteristics,oxidation resistance, heat transfer and dissipation capabilities, andmechanical and thermal loading parameters. Other suitable materialsinclude, for example, stainless steel or iron. Suitable materials forrotor 36, i.e., the portions of rotor 36 that are rubbed by snubbers 34,may include, for example, any suitable metallic (e.g., nickel basedalloys and/or iron-based alloys), intermetallic and/or compositematerial. In some embodiments, one or more coatings, e.g., such asalcrona (AICrN) and/or other coatings may be employed on rotor 36, e.g.,to reduce friction and wear, depending on the needs of the particularapplication. In some embodiments, rotor 36 may include a portion madefrom, and/or may be layered with and/or otherwise treated with a lowfriction material, an example of which is Graphalloy®. In someembodiments, the materials for snubbers 34 and rotor 36 may be selectedto have a similar coefficient of thermal expansion. In one form, the useof snubbers 34 to share rotor 36 loads with foil bearing 28 allows foilbearing 28 to be sized for a lower design load, while retaining thecapability to handle short duration transient peak loads. By being sizedfor a lower design load than that which would be required absent the useof snubbers 34, foil bearing 28 may be smaller and lighter thanotherwise.

Embodiments of the present invention include a gas turbine engine,comprising: a compressor; a turbine; a static structure; a foil bearingsystem operative to transmit rotor loads from at least one of thecompressor and the turbine to the static structure, wherein the foilbearing system includes a foil bearing and a self-aligning foil bearingmount coupled to the foil bearing, wherein the self-aligning foilbearing mount is operative to align the foil bearing with an axis ofrotation of the at least one of the compressor and the turbine; and asnubber operative to transmit rotor loads from at least one of thecompressor and the turbine to the static structure in parallel with thefoil bearing.

In a refinement, the self-aligning foil bearing mount includes a staticcomponent and a movable component, and wherein the self-aligning foilbearing mount is operable to self-align by displacing the movablecomponent relative to the static component.

In another refinement, the movable component is operable to rotaterelative to the static component.

In yet another refinement, rotation of the movable component is rotationof the entire movable component.

In still another refinement, the static component includes a firstsurface; wherein the movable component includes a second surface insliding contact with the first surface; and wherein displacement of themovable component relative to the static component includes the secondsurface sliding against the first surface.

In yet still another refinement, self-alignment is not achieved byflexure of the self-aligning foil bearing mount.

In a further refinement, the self-aligning foil bearing mount is aspherical bearing.

In a yet further refinement, the engine includes a shaft coupled to atleast one of the compressor and the turbine, wherein rotor loads aretransmitted from the shaft through the foil bearing, and from the foilbearing to the static structure via the selfaligning foil bearing mount.

In a still further refinement, the shaft couples the compressor to theturbine.

In a yet still further refinement, the engine includes a rotatingjournal and a housing disposed opposite to the rotating journal, whereinthe snubber is operative to limit the proximity of the housing relativeto the journal.

Embodiments of the present invention include a gas turbine engine,comprising: a rotor; a static structure; a foil bearing system operativeto transmit rotor loads from the rotor to the static structure, whereinthe foil bearing system includes: a foil bearing; and a self-aligningfoil bearing mount operative to align the foil bearing with an axis ofrotation of the rotor, wherein the self-aligning foil bearing mount hasa static component and a movable component; and wherein theself-aligning foil bearing mount is operable to self-align with the axisof rotation of the rotor by sliding displacement of the movablecomponent relative to the static component.

In a refinement, the self-aligning foil bearing mount is a sphericalbearing.

In another refinement, the engine further includes a snubber operativeto transmit rotor loads from the rotor to the static structure inparallel with the foil bearing.

In yet another refinement, the engine also includes a bearing housingfor housing the foil bearing, wherein the snubber is positioned withinthe bearing housing.

In still another refinement, the snubber is positioned adjacent to thebearing housing.

In yet still another refinement, the snubber is formed of a compositematerial.

In a further refinement, the foil bearing is a compliant foil radial airbearing.

Embodiments of the present invention include a foil bearing system for agas turbine engine, comprising: a foil bearing operative to transmitrotor loads from a rotor of the gas turbine engine to a static structureof the gas turbine engine; and means for aligning the foil bearing withan axis of rotation of the rotor.

In a refinement, the foil bearing system further includes means forsharing the rotor loads with the foil bearing.

In another refinement, the means for aligning includes a sphericalbearing.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment(s), but on the contrary, is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims, which scope is to be accordedthe broadest interpretation so as to encompass all such modificationsand equivalent structures as permitted under the law. Furthermore itshould be understood that while the use of the word preferable,preferably, or preferred in the description above indicates that featureso described may be more desirable, it nonetheless may not be necessaryand any embodiment lacking the same may be contemplated as within thescope of the invention, that scope being defined by the claims thatfollow. In reading the claims it is intended that when words such as“a,” “an,” “at least one” and “at least a portion” are used, there is nointention to limit the claim to only one item unless specifically statedto the contrary in the claim. Further, when the language “at least aportion” and/or “a portion” is used the item may include a portionand/or the entire item unless specifically stated to the contrary.

What is claimed is:
 1. In a foil bearing system for transmitting rotorloads from a rotor to a static structure having: a foil bearingsurrounding a rotatable shaft; a housing surrounding the foil bearing;and a bearing mount affixed to the static structure and surrounding thehousing, a method of self-aligning the foil bearing with the axis ofrotation of the shaft comprising: providing a crown extending radiallyoutward having a convex surface circumferentially disposed about aportion of the housing; providing a receiver having a concave surfacecircumferentially disposed about a portion of the bearing mount; andslideably mating the convex surface of the crown with a concave surfaceof the bearing mount to thereby permit rotation of the housing about anaxis that is perpendicular to the axis of rotation of the shaft.
 2. Themethod of claim 1 wherein the rotor is at least one of a compressorrotor or a turbine rotor.
 3. The method of claim 1 wherein the convexsurface of the crown is spherical in axial cross-section.
 4. The methodof claim 3 wherein the foil bearing is preloaded against the shaft. 5.The method of claim 4 wherein the crown is integral with the housing. 6.The method of claim 5 wherein said receiver is split.
 7. A foil bearingsystem for a turbine engine comprising: at least one foil bearing and ahousing disposed between a rotor of the turbine engine and a staticstructure of the turbine engine, said housing having a convex crowncircumferentially disposed about an axis of rotation of said rotor, abearing mount affixed to said static structure, said bearing mounthaving a concave receiver, wherein said convex crown is slideably matedwith said concave receiver to thereby permit rotation of said housingabout an axis that is perpendicular to the axis of rotation of saidrotor to thereby self-align said foil bearing to the axis of rotation ofsaid rotor.
 8. The system of claim 7 wherein said convex crown and saidconcave receiver comprise spherical surfaces in axial cross-section. 9.The system of claim 7 wherein a radially-outward surface of said convexcrown has a friction reducing coating.
 10. The system of claim 7 whereinsaid at least one foil bearing comprises a bump foil and a top foil. 11.The system of claim 10 wherein said top foil is preloaded against saidrotor.
 12. The system of claim 7 wherein said housing is adapted torotate about an axis that is perpendicular to the axis of rotation ofsaid rotor to thereby effect self-alignment of said convex crown to saidconcave receiver.
 13. The system of claim 8 wherein the convex crown isformed separately from and affixed to the housing.
 14. A systemcomprising: a rotor; a static structure; a foil bearingcircumferentially disposed about said rotor; a housing circumferentiallydisposed about said foil bearing, said housing comprising a convex crownextending radially outward; and a bearing mount affixed to said staticstructure circumferentially disposed about said housing, said bearingmount comprising a concave recess, wherein said housing is positioned sothat said convex crown is mated with said concave recess to permitsliding displacement between said housing and said bearing mount tothereby permit rotation of said housing about an axis that isperpendicular to the axis of rotation of said rotor.
 15. The system ofclaim 14 wherein said convex crown and said concave recess comprisespherical surfaces in axial cross-section.
 16. The system of claim 15wherein said bearing mount is split.
 17. The system of claim 16 whereinsaid bearing mount is secured to said static structure by a threadednut.
 18. The system of claim 15 wherein said bearing mount is integralwith said static structure.
 19. The system of claim 15 wherein saidconvex crown is integral to said housing.
 20. The system of claim 19wherein said foil bearing is preloaded against said rotor.